make _request() return {} if reply.read() returns bad json

This should fix errors like this:

p.imageCreate(imm,"8ded48362d1611e4b18dab8e5be14f3c")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "VscApiClient/__init__.py", line 657, in imageCreate
    self._request('PUT', url_path, {'create': 1}, data)
  File "VscApiClient/__init__.py", line 884, in _request
    return json.loads(reply_data)
  File "/usr/lib/python2.7/json/__init__.py", line 326, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 360, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 378, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded